Projection exposure apparatus and method

ABSTRACT

A projection exposure apparatus has an illumination optical system for illuminating a mask, on which a predetermined pattern is formed, with light from a light source, a projection optical system for forming an image of the pattern of the mask on a photosensitive substrate, a mask stage for holding the mask and moving the mask within a plane perpendicular to the optical axis of the projection optical system, a substrate stage for moving the photosensitive substrate within a plane conjugate to the plane with respect to the projection optical system, and an imaging characteristic correction system for correcting an imaging characteristic of the projection optical system. The apparatus synchronously moves the mask and the photosensitive substrate along the optical axis of the projection optical system so as to expose the entire pattern surface of the mask. The apparatus includes an incident light intensity input system for inputting the intensity of the illumination light, which is incident on the projection optical system through the mask, in accordance with the position of the mask, and an imaging characteristic calculation device for calculating a variation in imaging characteristic of the projection optical system on the basis of information from the incident light intensity input system. The imaging characteristic correction system is controlled on the basis of a result obtained by the imaging characteristic calculation device.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a projection exposure apparatusand method and, more particularly, to a scan type projection exposureapparatus and method used to manufacture semiconductor integratedcircuits and liquid crystal devices.

[0003] 2. Related Background Art

[0004] Many conventional apparatuses of this type have correctionfunctions for imaging characteristics because the apparatuses need tomaintain high imaging characteristics. Factors which cause the imagingcharacteristics to vary are changes in external environment such asatmospheric pressure and temperature, and slight absorption of exposurelight by a projection optical system. With regard to changes inenvironment, the atmospheric pressure and the like are monitored bysensors, and correction is performed in accordance with the detectionvalues, as disclosed in, e.g., U.S. Pat. No. 4,687,322. With regard toabsorption of exposure light, light energy incident on a projectionoptical system is measured, and a change in imaging characteristic owingto absorption of exposure light is calculated on the basis of themeasurement value, thereby performing correction, as disclosed in, e.g.,U.S. Pat. No. 4,666,273. In this known method, light energy incident onthe projection optical system through a mask is detected by, e.g., aphotoelectric sensor arranged on a substrate stage. In addition to lightenergy for projection exposure, which is incident from the mask side,light energy is incident on the projection optical system after it isreflected by a photosensitive substrate. This light energy also changesthe imaging characteristics of the projection optical system dependingon the intensity. With regard to such light energy, for example, asdisclosed in U.S. Pat. No. 4,780,747, light reflected by aphotosensitive substrate is measured by a photoelectric sensor arrangedin an illumination optical system. The sensor receives the light througha projection optical system and a mask, and a total change in imagingcharacteristic is calculated in consideration of a change in imagingcharacteristic owing to this reflected light energy. In this method,light reflected by an optical member, a mask pattern, and the like isincident on the photoelectric sensor in the illumination optical systemtogether with light reflected by the substrate. For this reason, aplurality of reference reflecting surfaces having different knownreflectances are set on a substrate stage, and the ratio of therespective outputs from the photoelectric sensor, which correspond tothe reference reflecting surfaces, is obtained in advance. Thereflectance (more accurately, reflection intensity) of thephotosensitive substrate is obtained on the basis of this ratio. Asdescribed above, since light reflected by a mask pattern is superposedon light reflected by a photosensitive substrate, sensor outputscorresponding to a plurality of reference reflecting surfaces must beobtained every time a mask is replaced. Alternatively, sensor outputsmust be measured and registered in advance.

[0005] Conventionally, the amount of change in imagining characteristicowing to absorption of exposure light is obtained to perform correctionby the above-described methods.

[0006] The above conventional scheme has been developed on the basis ofa scheme of projecting/exposing the entire mask pattern on aphotosensitive substrate (called a batch exposure scheme or a full fieldscheme). Recently, however, a so-called scan exposure scheme has beendeveloped, in which exposure is performed by illuminating a portion of apattern area on a mask with a slit-like beam while moving the mask and aphotosensitive substrate relative to each other. In this scheme, sincethe illumination area on a mask is smaller than that in the batchexposure scheme, the amount of image distortion or illuminanceirregularity is small. Furthermore, no limitations are imposed on thefield size of a projection optical system in the scan direction, andhence large-area exposure can be performed.

[0007] In a scan type exposure apparatus, however, energy incident onthe projection optical system changes while a mask is scanned withrespect to a slit-like illumination beam. For example, such a changeoccurs because the area of a light-shielding portion (a chromium layerof a pattern) formed on a mask changes in accordance with the positionof a slit illumination area on the mask, and hence the amount of energyincident on the projection optical system during a scan exposureoperation changes.

[0008] In addition, the amount of light reflected by a mask patternchanges in accordance with the position of a mask. Therefore, thedetection precision with respect to the amount of energy which isreflected by a photosensitive substrate and incident on the projectionoptical system inevitably deteriorates in the conventional scheme.

[0009] For the above-described reasons, in the conventional schemescorrection based on an accurate amount of change in imagingcharacteristic owing to absorption of exposure light cannot be performed

SUMMARY OF THE INVENTION

[0010] It is an object of the present invention to provide a projectionexposure apparatus of a scan exposure scheme, which can properly correctthe imaging characteristics.

[0011] In order to achieve the above object, according to a first aspectof the present invention, there is provided a projection exposureapparatus having an illumination optical system for illuminating a mask,on which a predetermined pattern is formed, with light from a lightsource, a projection optical system for forming an image of the patternof the mask on a photosensitive substrate, a mask stage for holding themask and moving the mask within a plane perpendicular to an optical axisof the projection optical system, a substrate stage for moving thephotosensitive substrate within a plane conjugate to the plane withrespect to the projection optical system, and imaging characteristiccorrection means for correcting an imaging characteristic of theprojection optical system, the apparatus synchronously moving the maskand the photosensitive substrate along an optical axis of the projectionoptical system so as to expose an entire pattern surface of the mask,and the apparatus including:

[0012] incident light intensity input means for inputting an intensityof the illumination light, which is incident on the projection opticalsystem through the mask, in accordance with a position of the mask,

[0013] imaging characteristic calculation means for calculating avariation in imaging characteristic of the projection optical system onthe basis of information from the incident light intensity input means;and

[0014] control means for controlling the imaging characteristiccorrection means on the basis of a result obtained by the imagingcharacteristic calculation means.

[0015] According to a second aspect of the present invention, there isprovided a projection exposure apparatus having an illumination opticalsystem for illuminating a mask, on which a predetermined pattern isformed, with light from a light source, a projection optical system forforming an image of the pattern of the mask on a photosensitivesubstrate, a mask stage for holding the mask and moving the mask withina plane perpendicular to an optical axis of the projection opticalsystem, a substrate stage for moving the photosensitive substrate withina plane conjugate to the plane with respect to the projection opticalsystem, and imaging characteristic correction means for correcting animaging characteristic of the projection optical system, the apparatussynchronously moving the mask and the photosensitive substrate along anoptical axis of the projection optical system so as to expose an entirepattern surface of the mask, and the apparatus including:

[0016] incident light intensity input means for inputting an intensityof the illumination light, which is incident on the projection opticalsystem through the mask, in accordance with a position of the mask;

[0017] reflected light intensity input means for inputting an intensityof the illumination light, which is reflected by the photosensitivesubstrate and incident on the projection optical system again, inaccordance with a position of the mask;

[0018] imaging characteristic calculation means for calculating avariation in imaging characteristic of the projection optical system onthe basis of information from the incident light intensity input meansand information from the reflected light intensity input means; and

[0019] control means for controlling the imaging characteristiccorrection means on the basis of a result obtained by the imagingcharacteristic calculation means.

[0020] According to the present invention, even if energy incident onthe projection optical system changes when a mask is scanned during anexposure operation, no problem is posed because illumination lightintensity data corresponding to the position of the mask can be used forcalculation of a variation in imaging characteristic caused byabsorption of exposure light. In addition, according to the presentinvention, a variation in imaging characteristic owing to absorption ofexposure light can be accurately obtained because energy incident on theprojection optical system is calculated in consideration of informationabout light reflected by the photosensitive substrate.

[0021] As described above, according to the present invention, since avariation in imaging characteristic can be accurately calculated on thebasis of the amount of energy incident on the projection optical systemwhich changes in accordance with the position of a mask, the imagingcharacteristic can be corrected without any error even in a scan typeexposure apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a schematic block diagram showing the arrangement of ascan type exposure apparatus according to an embodiment of the presentinvention;

[0023]FIG. 2 is a perspective view showing a scan/exposure operation inthe apparatus in FIG. 1;

[0024]FIG. 3 is a block diagram showing the detailed arrangement ofcomponents around the wafer stage of the apparatus in FIG. 1;

[0025]FIG. 4A is a graph showing incident energy;

[0026]FIG. 4B is a graph showing the relationship between the incidentenergy and the variation in magnification;

[0027]FIG. 5 is a graph showing a change in reticle transmittance in acase wherein the reticle is moved;

[0028]FIG. 6 is a graph showing the relationship between the reflectanceand the reference reflectances;

[0029]FIG. 7A is a graph showing the relationship between thereflectance and the reference reflectances in a case wherein the reticleis moved;

[0030]FIG. 7B is a graph showing a change in reticle reflectance in acase wherein the reticle is moved;

[0031]FIG. 8A is a graph showing the incident energy corresponding toeach reticle position in a case wherein the reticle is scanned;

[0032]FIG. 8B is a graph showing variations in incident energy andimaging characteristic in a case wherein the incident energy changes atthe respective positions (timings);

[0033]FIG. 9 is a plan view showing the relationship between a reticleblind viewed from above and a projection field;

[0034]FIG. 10 is a perspective view stereoscopically showing theilluminance distribution of illumination light; and

[0035]FIG. 11 is a graph showing the illuminance distribution in thescan direction

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] An embodiment of the present invention will be described belowwith reference to the accompanying drawings. FIG. 1 is a schematicrepresentation of the arrangement of a projection exposure apparatussuitable for an embodiment of the present invention. Illumination lightIL emitted from a light source 1 passes through a shutter 2 and isadjusted to a predetermined beam diameter by a lens system 4 constitutedby a collimator lens and the like. The illumination light IL is thenincident on a fly-eye lens 6 through a mirror 5. The illumination lightIL is an excimer laser beam such as a KrF or ArF laser beam, a harmonicwave of a copper vapor laser or a YAG laser, or an ultraviolet line froma super-high pressure mercury lamp. The shutter 2 is inserted/removedin/from an optical path by a shutter driver 3 to control opening/closingof the optical path. If the light source 1 is a pulse light source suchas an excimer laser, the shutter 2 need not be used for light amountcontrol.

[0037] The light beam emerging from the fly-eye lens 6 is incident on areticle (mask) R, on which a semiconductor circuit pattern or the likeis drawn, through relay lenses 7 a and 7 b, a reticle blind B, a mirror9, and a condenser lens 10 The system constituted by the fly-eye lens 6,the relay lenses 7 a and 7 b, the mirror 9, and the condenser lens 10serves to superpose the illumination light IL emerging from therespective lens elements of the fly-eye lens 6 on the reticle R toilluminate the reticle R with a uniform light intensity. Thelight-shielding surface of the reticle blind 8 is conjugate to thepattern area of the reticle R. The size (slit width or the like) of theopening portion of the reticle blind 8 is adjusted by opening/closing aplurality of movable light-shielding portions (e.g., two L-shapedmovable light-shielding portions) constituting the reticle blind 8 byusing a motor 11. By adjusting the size of this opening portion, anillumination area IA for illuminating the reticle R is arbitrarily set.The reticle R is vacuum-chucked on a reticle stage RST disposed on abase 12. The reticle stage RST can be finely moved on the base 12two-dimensionally through an air bearing and the like to position thereticle R within a plane perpendicular to an optical axis IX of theillumination system. The reticle stage RST can also be moved on the base12 in a predetermined direction (scan direction) by a reticle driver 13constituted by a linear motor and the like. The reticle stage RST has atleast a moving stroke which allows the entire surface of the reticle Rto cross the optical axis IX of the illumination system. A movablemirror 15 for reflecting a laser beam from an interferometer 14 is fixedto an end portion of the reticle stage RST. The position of the reticlestage RST in the scan direction is always detected by the interferometer14 with a resolving power of about 0.01 μm. Position information aboutthe reticle stage RST, which is obtained by the interferometer 14, issupplied to a control system 16. The control system 16 controls thereticle driver 13 to move the reticle stage RST on the basis of theposition information about the reticle stage RST. The initial positionof the reticle stage RST is determined such that the reticle R ispositioned to a reference position with high precision by a reticlealignment system. Therefore, the position of the reticle R can bemeasured with sufficiently high precision by only measuring the positionof the movable mirror 15 using the interferometer 14.

[0038] The illumination light IL passing through the reticle R isincident on, e.g., a double side telecentric projection optical systemPL. The projection optical system PL then forms a projection image,obtained by reducing the circuit pattern of the reticle R to ⅕ or ¼, ona wafer W having a resist (photosensitive agent) coated on its surface

[0039] In the exposure apparatus of this embodiment, as shown in FIG. 2,the reticle R is illuminated with the rectangular (slit-like)illumination area IA whose longitudinal direction is perpendicular tothe reticle-side scan direction (+x direction), and the reticle R isscanned at a speed indicated by an arrow Vr in an exposure operation.The illumination area IA (whose center almost coincides with the opticalaxis IX) is projected on the wafer W through the projection opticalsystem PL to form a projection area IA′. Since the wafer W and thereticle R have an inverted imaging relationship, the wafer W is scannedat a speed indicated by an arrow Vw in the opposite direction (−xdirection) to the direction indicated by the arrow Vr in synchronismwith the reticle R, thereby allowing the entire surface of a shot areaSA of the wafer W to be exposed. A scan speed ratio Vw/Vr accuratelycorresponds to the reducing ratio of the projection optical system PL sothat the pattern of a pattern area PA of the reticle R can be accuratelyreduced/transferred onto the shot area SA. The longitudinal dimension ofthe illumination area IA is set to be larger than that of the patternarea PA and smaller than the maximum width of a light-shielding area ST.By scanning the illumination area IA, the entire surface of the patternarea PA can be illuminated.

[0040] Referring to FIG. 1 again, the wafer W is vacuum-chucked on awafer holder 17 and held on a wafer stage WST through the wafer holder17. The wafer holder 17 can be inclined in an arbitrary direction withrespect to the optimum imaging plane of the projection optical system PLand can be finely moved along the optical axis IX (z direction) by adriver (not shown). In addition, the wafer stage WST is designed to bemoved not only in the scan direction (x direction) but also in adirection (y direction) perpendicular to the scan direction to bearbitrarily moved to a plurality of shot areas so as to allow astep-and-scan operation. That is, the wafer stage WST repeats anoperation of scanning/exposing a given shot area on the wafer W and anoperation of moving to the next shot exposure start position. A waferstage driver 18 constituted by a motor and the like serves to move thewafer stage WST in the X and y directions. A movable mirror 20 forreflecting a laser beam from an interferometer 19 is fixed to an endportion of the wafer stage WST. The X- and Y-positions of the waferstage WST are always detected by the interferometer 19 with a resolvingpower of about 0.01 μm. Position information (or speed information)about the wafer stage WST is supplied to a wafer stage controller 21.The wafer stage controller 21 controls the wafer stage driver 18 on thebasis of this position information (or speed information)

[0041] The wafer W which has been exposed and processed is aligned by awafer alignment system (not shown) such that the projection image of thereticle is accurately superposed and exposed on the wafer W. A detaileddescription of this operation will be omitted.

[0042] In the apparatus shown in FIG. 1, an oblique incident type waferposition detection system (focus detection system) constituted by aradiation optical system 22 and a reception optical system 23 is fixedto a support portion (column) 24 for supporting the projection opticalsystem PL. The radiation optical system 22 radiates an imaging lightbeam for forming a pinhole or a slit image onto the optimum imagingplane of the projection optical system PL from a direction oblique tothe optical axis IX. The reception optical system 23 receives a lightbeam, of the imaging light beam, which is reflected by the surface ofthe wafer W through a slit. The arrangement and the like of this waferposition detection system are disclosed in, e.g., U.S. Pat. No.4,650,983. The system is used to detect the positional deviation of thewafer surface in the vertical direction (z direction) with respect tothe imaging plane so as to drive the wafer holder 17 in the z directionto keep a predetermined distance between the wafer W and the projectionoptical system PL. Wafer position information from the wafer positiondetection system is input to a focus position controller 25. This waferposition information is supplied to the wafer stage controller 21through a main control system 100. The wafer stage controller 21 drivesthe wafer holder 17 in the z direction on the basis of the waferposition information.

[0043] Assume that in this embodiment, calibration of the wafer positiondetection system is performed in advance by adjusting the angle of aplane parallel glass (plane parallel) (not shown) arranged in theradiation optical system 22 such that the imaging plane becomes a zeroreference. Alternatively, the inclination angle of a predetermined areaon the wafer W with respect to the imaging plane may be detected byusing a horizontal position detection system like the one disclosed inU.S. Pat. No. 4,558,949, or by designing a wafer position detectionsystem to detect focus positions at a plurality of arbitrary positionsin the image field of the projection optical system PL (e.g., by forminga plurality of slit images in the image field).

[0044] A radiation amount sensor 41 is disposed on the wafer stage WSTat almost the same level as that of the surface of the wafer W. Theradiation amount sensor 41 has a light-receiving surface which is atleast larger than the projection area IA′. In measurement, the radiationamount sensor 41 is moved to a position immediately below the opticalaxis IX of the projection optical system PL, and outputs a signal Sccorresponding to the total intensity of illumination light passingthrough the reticle R. This signal Sc is used for initialization incorrecting the imaging characteristics which vary upon incidence ofillumination light, as will be described in detail later.

[0045] The arrangement of the interferometer 19 will be described indetail below with reference to FIG. 3. FIG. 3 shows the detailedarrangement of components around the wafer stage WST. The interferometer19 in this embodiment is constituted by five interferometers, i.e., Xinterferometers (interferometers 19 x ₁ and 19 x ₂) for measuring theX-position of the wafer stage WST, Y interferometers (interferometers 19y ₁ and 19 y ₂) for measuring the Y-position of the wafer stage WST, andan alignment interferometer 19 ya having an optical axis extendingthrough a center OAc of an observation area OA of an off-axis alignmentsystem (not shown) in the y direction. The interferometers 19 x ₁ and 19x ₂ are arranged to be symmetrical with respect to a straight line Cxextending through a center Ce of a projection field if of the projectionoptical system PL in a direction parallel to the X axis. A movablemirror 20 x is an X-position detection movable mirror for reflectinglaser beams from the interferometers 19 x ₁ and 19 x ₂. Theinterferometers 19 y ₁ and 19 y ₂ are arranged to be symmetrical withrespect to a straight line Cy extending through the center Ce of theprojection field if of the projection optical system PL in a directionparallel to the Y axis. A movable mirror 20 y is a Y-position detectionmovable mirror for reflecting laser beams from the interferometers 19 y₁ and 19 y ₂. The wafer stage controller 21 incorporates a positioncalculator 21Xe for calculating an X-position, a yawing calculator 21Xθfor obtaining the yawing amount of the movable mirror 20 x (wafer stageWST) from the Y-axis, a position calculator 21Ye for calculating aY-position, a yawing calculator 21Yθ for obtaining the yawing amount ofthe movable mirror 20 y (wafer stage WST) from the X-axis, and aposition calculator 21Ya for calculating the Y-position of the off-axisalignment system at the center OAc. The position calculator 21Xecalculates an X-position measurement value Xe of the wafer stage WST onthe basis of the average of measurement values obtained by theinterferometers 19 x ₁ and 19 x ₂. The yawing calculator 21Xθ calculatesa yawing amount Xθ in the movement of the wafer stage WST in the xdirection on the basis of the difference between the measurement valuesobtained by the interferometers 19 x ₁ and 19 x ₂. The positioncalculator 21Ye calculates a Y-position measurement value Ye of thewafer stage WST on the basis of the average of measurement valuesobtained by the interferometers 19 y ₁ and 19 y ₂. The yawing calculator21Yθ calculates a yawing amount Yθ in the movement of the wafer stageWST in the y direction on the basis of the difference between themeasurement values obtained by the interferometers 19 y ₁ and 19 y ₂.

[0046] The position calculator 21Ya serves to measure a Y-position Ya ofthe wafer stage WST when a mark on the wafer W is to be detected by theoff-axis alignment system. The alignment position measurement system(the interferometer 19 ya and the position calculator 21Ya) is arrangedto prevent an Abbe's error in a mark detecting operation which is causedwhen the observation center OAc of the off-axis alignment system isdeviated from the center Ce of the projection field if of the projectionoptical system PL in the x direction. A reference plate FM having areference mark formed thereon is arranged on the wafer stage WST. Forexample, the reference plate FM is used to measure the distance(baseline) between the observation center OAc of the off-axis alignmentsystem and the center Ce of the projection field if of the projectionoptical system PL. The reference plate FM has a reflecting surface R₂having a reflectance r₂, and a reflecting surface R₃ having an almostzero reflectance. The surface of the radiation amount sensor 41 has areflecting surface R₁ having a reflectance r₁. The respective reflectingsurfaces are used to obtain offset components or used as referencereflecting surfaces for calculating the reflectance of a wafer, as willbe described later.

[0047] As shown in FIG. 3, the yawing amount of the wafer stage WST isindependently measured by using both the X-axis movable mirror 20 x andthe Y-axis movable mirror 20 y. In this measurement, an averagingcircuit 21 k is used to average the yawing amounts Xθ and Yθ measured bythe two mirrors 20 x and 20 y. With this operation, variations inmeasurement value, obtained by the X-axis interferometers 19 x ₁ and 19x ₂ and the Y-axis interferometers 19 y ₁ and 19 y ₂, owing to airfluctuations in the respective laser beam paths are averaged, allowingmeasurement of a yawing amount with higher reliability.

[0048] No significant problem is posed in the case of the wafer stageWST used for wafer exposure, as shown in FIG. 3. However, in the case ofa stage for exposing a glass plate for the manufacture of a liquidcrystal display element, the movement stroke of the stage may becomeextremely large in the X or y direction depending on the position of aprojection image (pattern arrangement) on the glass plate. In this case,on the side where the movement stroke is extremely large, the laser beampath of one of a pair of interferometers for measuring yawing amountsmay deviate from the movable mirror near the end point of the stroke.For this reason, it may be checked whether the laser beam path deviatesfrom the movable mirror on the X- or Y-axis side depending on a patternarrangement (which can be known in design prior to exposure), and ayawing amount measured by the interferometer on the axis side where thelaser beam path does not deviate from the movable mirror may beselectively used. As is apparent, when the laser beam paths of theinterferometers on the two axis sides do not deviate from the movablemirrors, an average yawing amount obtained by the averaging circuit 21 kis preferably used.

[0049] A beam splitter 26 for reflecting part (e.g., 5%) of theillumination light IL and transmitting the remaining part, is arrangedin the optical path between the fly-eye lens 6 and the reticle R in theapparatus shown in FIG. 1. The beam splitter 26 guides light reflectedby the reticle R to a reflected light sensor 27. As the reflected lightsensor 27, a photoelectric sensor such as a silicon photodiode or aphotomultiplier is used. The reflected light sensor 27 receives lightreflected by the wafer W through the reticle R and outputs a signal Sbto the main control system 100. Since it is preferable that thereflected light sensor 27 receive light reflected by the entireillumination area IA (IA′), the reflected light is preferably focused bya lens or the like, or the reflected light sensor 27 is preferablydisposed at a Fourier transform plane corresponding to the wafer W,i.e., a position conjugate to the pupil position of the projectionoptical system PL.

[0050] The beam splitter 26 guides part of illumination light from thelight source 1 to a photoelectric sensor 28 for detecting the intensityof a light beam from the light source 1. The photoelectric sensor 28receives part of the illumination light IL reflected by the beamsplitter 26 and outputs an output signal Sa to the main control system100.

[0051] The functions of the reflected light sensor 27 and thephotoelectric sensor 28 will be described in detail later.

[0052] The apparatus of this embodiment also includes an input means 101such as a keyboard or a bar code reader and hence can input variousinformation, e.g., thermal time constant information about theprojection optical system, transmittance information about the reticleR, an illumination slit width, a target exposure amount, and a scanspeed.

[0053] The exit end face of the fly-eye lens 6, on which a plurality oftwo-dimensional light source images are formed, has a relationship ofFourier transform with the pattern area of the reticle R. An aperturestop 29 for changing the shape of a two-dimensional light source isdisposed near this exit end faces. As the aperture stop 29, an annularaperture stop for limiting the shape of a two-dimensional light sourceimage to an annular shape, an aperture stop for limiting the shape of atwo-dimensional light source image to a plurality of discrete areasdecentered from the optical axis IX, or a circular aperture stop forchanging the size of a two-dimensional light source image withoutchanging the position of the center may be used. Annular aperture stopsare disclosed in Japanese Laid-Open Patent Application No. 61-91662 andthe like. As an aperture stop for limiting the shape of atwo-dimensional light source image, for example, an aperture stop havingfour opening portions arranged to be point symmetrical about the opticalaxis IX is disclosed in detail in Japanese Laid-Open Patent ApplicationNo. 4-225514.

[0054] The apparatus shown in FIG. 1 includes a correction mechanism forcorrecting the imaging characteristics of the projection optical systemPL. This correction mechanism for imaging characteristics will bedescribed below.

[0055] As shown in FIG. 1, in this embodiment, the opticalcharacteristics of the projection optical system PL itself and itsprojection image imaging characteristics can be corrected byindependently driving the reticle R and lens elements 34 and 35 using animaging characteristic controller 30. The reticle stage RST can befinely moved along the optical axis IX (in the vertical direction) bydriving elements 31. As the driving elements 31, piezoelectric elements,electrostrictive elements, or air dampers are used. Three or fourdriving elements 31 are used to drive the whole reticle stage RST.

[0056] Specifications of the imaging characteristics of the projectionoptical system PL (i.e., imaging characteristics of the image of apattern of the reticle exposed to the wafer) include a focus position(imaging plane position), a projecting magnification, a distortion, acurvature of field, an astigmatism, and the like. These values can beindependently corrected. In this embodiment, however, for the sake of asimple explanation, correction of a focus position, a projectingmagnification, and a curvature of field in the double side telecentricprojection optical system will be described below with reference to amethod of driving the lens elements of the projection optical system PL.

[0057] The first group lens element 34 located nearest to the reticle Ris fixed to a support member 36, and the second group lens element 35 isfixed to a support member 37. A lens element below a lens element 38 isfixed to a mirror barrel portion 39 of the projection optical system PL.Assume that in this embodiment, the optical axis IX of the projectionoptical system PL is the optical axis of the lens element fixed to themirror barrel portion 39 The support member 36 is coupled to the supportmember 37 through a plurality of (e.g., three; two are shown in FIG. 1)extendible driving elements 32. The support member 37 is coupled to themirror barrel portion 39 through a plurality of extendible drivingelements 33.

[0058] In this arrangement, when the lens elements 34 and 35 aretranslated along the optical axis, a projecting magnification (theenlargement/reduction amount of the size of a projection image) M, acurvature of field C, and a focus position F slightly change in amountat change rates corresponding to the moving amounts. Letting z₁ be thedriving amount of the lens element 34 and z₂ be the driving amount ofthe lens element 35, variations ΔM, ΔC, and ΔF of the projectingmagnification M, the curvature of field C, and the focus position F areexpressed by the following equations, respectively:

ΔM=C _(M1) ×z ₁ +C _(M2) ×z ₂  (1)

ΔC=C _(C1) ×z ₁ +C _(C2) ×z ₂  (2)

ΔF=C _(F1) ×z ₁ +C _(F2) ×z ₂  (3)

[0059] where C_(M1), C_(M2), C_(C1), C_(C2), C_(F1), and C_(F2) areconstants representing the change rates of variations with respect tothe driving amounts of the respective lens elements.

[0060] As described above, the wafer position detection systems 22 and23 serve to detect the shift amount of a wafer surface with respect tothe optimum focus position, of the projection optical system PL, whichserves as a zero reference. Therefore, when a proper offset amount z₃ iselectrically or optically given to the wafer position detection systems22 and 23, a focus position shift caused upon driving of the lenselements 34 and 35 can be corrected by positioning the wafer surfaceusing the wafer position detection systems 22 and 23. In this case,equation (3) is rewritten as follows:

ΔF=C _(F1) ×z ₁ +C _(F2) ×z ₂ +z ₃  (4)

[0061] As described above, the variations ΔM, AC, and AF can bearbitrarily corrected by setting the driving amounts z₁ to z₃ accordingto equations (1), (2), and (4). In this case, three types of imagingcharacteristics are simultaneously corrected. If, however, the variationin imaging characteristic, of the optical characteristics of theprojection optical system, which is caused by absorption of illuminationlight is negligible, the corresponding correction described above neednot be performed. In addition, in this embodiment, if an imagingcharacteristic other than the three types of imaging characteristicsdescribed above greatly changes, correction must be performed withrespect to that imaging characteristic. In this embodiment, since thevariation in curvature of field is corrected to zero or an allowablevalue or less, no special correction of the astigmatism is performed.

[0062] In this embodiment, the variation ΔF in focus position (equation(4)) is corrected as follows. For example, a proper offset amount iselectrically or optically (using a plane parallel) given to the waferposition detection systems 22 and 23, and the wafer W is moved in the zdirection by using the wafer position detection systems 22 and 23,thereby setting the surface of the wafer W at the optimum imaging plane(best focus position) of the projection optical system PL.

[0063] In this embodiment, the reticle R and the lens elements 34 and 35are moved along the optical axis by the imaging characteristiccontroller 30. Especially the lens elements 34 and 35 greatly influencethe respective characteristics associated with magnification,distortion, and curvature of field (astigmatism) and can be easilycontrolled, as compared with other lens elements. In this embodiment,the two groups of movable lens elements are arranged. However, three ormore groups of lens elements may be arranged. In this case, the movingrange of each lens element can be broadened while variations in otheraberrations are suppressed. In addition, this arrangement can properlycope with various types of distortions (trapezoidal and rhombicdistortions) and a curvature of field (astigmatism). Furthermore,distortions and the like can be corrected by driving the reticle R inthe z direction.

[0064] Feedback control is also performed with respect to apredetermined target position by using position sensors for monitoringdriving amounts, e.g., encoders, capacitive sensors, and reflection typesensors. When the above mechanism is to be used only for maintenance,even if dynamic correction is not performed during an exposureoperation, the mechanism may be replaced with a fine feed mechanism witha micrometer head or a semi-stationary mechanism with a washer.

[0065] In the above imaging characteristic correction mechanism,correction is performed by moving the reticle R and the elements.However, this embodiment may use any proper correction mechanism of adifferent scheme other than that described above. For example, thefollowing method may be used. A space defined by two lens elements orplane parallel glasses is sealed, and the pressure in the sealed spaceis adjusted. The apparatus shown in FIG. 1 includes a pressure controlsystem 40 for adjusting the pressure in the sealed space defined by thelens elements so as to finely correct the optical characteristics(especially the magnification) of the projection optical system PLitself. The pressure control system 40 is also controlled by the imagingcharacteristic controller 30 to provide desired imaging characteristicsfor a projection images. Since the detailed arrangement of the pressurecontrol system 40 is disclosed in U.S. Pat. No. 4,871,237, a descriptionthereof will be omitted.

[0066] As described above, variations in the imaging characteristics ofthe projection optical system PL owing to absorption of exposure lightcan be satisfactorily corrected by driving the lens elements or usingthe correction mechanism for adjusting the pressure in the sealed spacedefined by the lens elements.

[0067] A method of calculating a variation in imaging characteristicowing to absorption of exposure light will be described next. Theabove-described imaging characteristic correction mechanism is optimallydriven on the basis of the calculated variation in imagingcharacteristic. Strictly speaking, variations in the above imagingcharacteristics need to be separately calculated. This is because thedegrees to which the respective imaging characteristics are influencedslightly differ from each other depending on the lens elementsconstituting the projection optical system PL, and hence variationcharacteristics differ even if illumination light having the same energyis incident on the projection optical system PL. However, the basiccalculation methods are the same, but the coefficients used in thecalculations of the respective characteristics are slightly differentfrom each other. Therefore, for the sake of simplicity, the followingdescription is made with reference to the variation ΔM in projectingmagnification.

[0068] The principle of the method will be described first. Thevariation ΔM in projecting magnification is caused because therefractive indexes or curvatures of the lens elements in the projectionoptical system PL slightly change when the lens elements slightly absorbillumination light and increase in temperature. Consider one lenselement. Energy of illumination light is input to the lens element,i.e., heat is absorbed thereby, and at the same time, heat is dissipatedto external components such as the mirror barrel portion 39. Thetemperature of the lens element is determined by the balance between theabsorption and dissipation of heat. Providing that the temperature riseand the variation ΔM in magnification are proportional to each other, itcan be considered that the variation ΔM in magnification is determinedby the heat balance. In general, when the temperature of the lenselement is low, absorption of heat is higher in rate than dissipation ofheat, and hence the temperature gradually increases. When thetemperature of the lens element becomes high as compared with theambient temperature, dissipation of heat becomes higher in rate thanabsorption of heat. When the absorption of heat balances the dissipationof heat, the lens element reaches a saturation level to be set in anequilibrium state. If an exposure operation is stopped, heat isgradually dissipated, and the temperature of the lens element decreases.When the difference between the temperature of the lens element and theambient temperature becomes small, the speed of heat dissipationdecreases. This characteristic is generally called a first-ordertime-lag, which can be expressed by a first-order differential equation.FIGS. 4A and 4B show this state. FIG. 4A shows incident energy. FIG. 4Bshows a magnification variation characteristic obtained whenillumination light of a predetermined energy amount is radiated on theprojection optical system PL for a predetermined period of time. Thevariation characteristic shown in FIG. 4B indicates a final variationΔM₁ (saturation level) in projecting magnification with respect toradiation energy E₁. The variation ΔM₁ in projecting magnification canbe determined by two values, i.e., a change rate ΔM₁/E₁ and a timeconstant T representing a change over time. Referring to FIG. 4B, thetime constant T can be defined as a time during which the magnificationchanges by ΔM₁ ×(1−e⁻¹) with respect to the final variation ΔM₁. In thiscase, when the change rate ΔM₁/E₁ and the time constant T are obtained,the variation ΔM in magnification can be calculated from an estimatedvalue of the energy E which is incident on the projection optical systemPL in accordance with the output Sa from the photoelectric sensor 28.More specifically, by always monitoring the incident energy E, thevariation ΔM can be sequentially calculated in the main control system100 on the basis of the change rate ΔM₁/E₁ and the time constant T. Thechange rate ΔM₁/E₁ and the time constant T can be obtained by checking acharacteristic like the curve shown in FIG. 4B while experimentallykeeping radiating illumination light on the projection optical systemPL. In practice, however, since a plurality of lens elements are presentin the projection optical system PL, the overall magnification variationcharacteristic is expressed by the sum of several first-order time-lagcharacteristics. The change rate ΔM₁/E₁ and the time constant T areinput to the main control system 100 through the input means 101. Asdescribed above, the change rate ΔM₁/E₁ and the time constant T arecoefficients of a first-order differential equation. This differentialequation is sequentially solved by numerical analysis using a generaldigital calculator or the like. In this case, if calculationsynchronization is performed at predetermined time intervalssufficiently shorter than the time constant T, and the value of theenergy E incident on the projection optical system PL is sequentiallyobtained (calculated) in accordance with this calculationsynchronization, the ΔM at a given time point can be calculated by themain control system 100.

[0069] A method of obtaining different values of the incident energy Ein accordance with the position of a reticle and obtaining the variationcharacteristic of an imaging characteristic in a case wherein the energyamount changes during an exposure operation for one shot will bedescribed below.

[0070] A method of obtaining the energy E sequentially radiated on theprojection optical system PL will be described below. When energyincident on the projection optical system PL is to be considered, theamount of light which is reflected by a wafer and incident on theprojection optical system again must be considered in addition to theamount of light which is incident on the projection optical system PLthrough a reticle. In a scan type apparatus, since the reticle R isscanned with respect to the slit-like illumination area IA (i.e., theoptical axis of the projection optical system), the total area of thelight-shielding portion of the reticle R sequentially changes inaccordance with the scan position, and the energy E incident on theprojection optical system PL changes in amount in accordance with thescan position of the reticle. For this reason, the incident energy E maybe calculated by obtaining the sum of the amount of light which isincident on the projection optical system PL through the reticle and theamount of light which is reflected by the wafer and incident on theprojection optical system PL again, at time intervals Δt of, e.g.,several msec as sampling time intervals.

[0071] In this case, the amount of light which is incident on theprojection optical system PL through the reticle is obtained on thebasis of the output Sa from the photoelectric sensor 28, and the amountof light which is reflected by the wafer and incident on the projectionoptical system PL again is obtained on the basis of the output Sb fromthe reflected light sensor 27. However, the output Sb from the reflectedlight sensor 27 includes light amount information about light reflectedby the reticle R and optical members in the illumination optical system.For this reason, in this embodiment, reference reflection plates havingdifferent known reflectances are used, and reference reflection data forobtaining the reflection intensity of the wafer are obtained inaccordance with the scan position of the reticle. The actual reflectance(reflection intensity) of the wafer is then obtained in accordance withthe scan position of the reticle on the basis of the referencereflection data. In addition, the transmittance (transmitted lightamount) of the reticle is obtained in accordance with the scan positionof the reticle, and the energy E is obtained on the basis of thesepieces of information.

[0072] A method of obtaining the incident energy E by using the waferreflectance and the transmittance of the reticle which are obtained onthe basis of the reference reflection data will be described next.Letting P be the amount of light which is incident on the projectionoptical system PL through the reticle R, and r be the reflectance of thewafer W, the total amount of light incident on the projection opticalsystem PL, including an amount P·r of light which is reflected by thewafer W and incident on the projection optical system PL, can beexpressed by equation (5):

E=P×(1+r)  (5)

[0073] Letting η be the transmittance of the reticle R at the radiationposition, Ip be the illuminance of a light source per unit area, and Sbe the radiation area, the light amount P can be expressed as follows:

P=Ip×S×q  (6)

[0074] In this case, the illuminance Ip is the illuminance (without areticle) on the wafer W per unit area, and the area S is the area of theprojection area IA′ of the wafer W for the sake of convenience. Since itis essential to obtain the relationship between the variation ΔM and theenergy E, the light amount P may be defined on the reticle R or anyother places.

[0075] In performing a scan type exposure operation, since the amount oflight which is incident on the projection optical system PL through thereticle R changes in accordance with the position of the reticle R, thereticle transmittance n must be obtained for each scan position of thereticle R. A method of obtaining the transmittance of a reticle will bedescribed below.

[0076] After the wafer stage WST is moved such that the radiation amountsensor 41 is located in the projection area IA′, only the reticle stageRST is scanned while the wafer stage WST is fixed and the reticle R isplaced on the reticle stage RST. At this time, the magnitude of anoutput Sc₁ from the radiation amount sensor 41 is sequentially read incorrespondence with the coordinate position (x_(R)) of theinterferometer 14 for measuring the position of the reticle stage RST.Similarly, the magnitude of the output Sa from the photoelectric sensor28 is read. A ratio Sc₁/Sa is then calculated and stored in a memory inthe main control system 100 in correspondence with each coordinateposition. For example, storage of such data in the memory (digitalsampling) may be performed at intervals corresponding to a predeterminedmoving amount (e.g., 0.01 μm to 10 μm) with reference to the resolvingpower (e.g. 0.01 μm) of the interferometer 14. In general, the maincontrol system 100 is constituted by a digital computer. For thisreason, in practice, several digital values of the ratio Sc₁/Sa, whichare sequentially calculated with a resolving power almost equal to theresolving power of the interferometer 14, may be averaged, and suchaverage values may be stored at position intervals (or time intervals)at which no problem is posed in terms of an error in the calculationprecision of a variation in magnification. Alternatively, the values ofthe ratio Sc₁/Sa, which are sequentially calculated with a resolvingpower almost equal to the resolving power of the interferometer 14 (or apredetermined moving amount larger than that thereof).

[0077] Note that the position where the reticle stage RST starts to moveso as to read the output Sc₁ is stored, as a reference position for aread operation, in the main control system 100. An output Sc₂ from theradiation amount sensor 41, reticle transmittance data η(x_(R)), theoutput Sb from the reflected light sensor 27, reference reflectance datarx(x_(R)), and offset component data, which output and data will bedescribed later, are all stored in the memory with reference to thisposition.

[0078] A ratio Sc₂/Sa′ (a constant value independent of the scanposition) between the output Sc₂ from the radiation amount sensor 41 andthe output Sa from the photoelectric sensor 28, which are detected atthe same timing before the reticle R is mounted on the reticle stageRST, is determined, and the data string (waveform) of the ratio Sc₁/Sastored in the memory is normalized (divided) by using the value of theSc₂/Sa′ as a denominator. With this operation, the data string of aratio Sc₁·Sa′/Sc₂·Sa output from the radiation amount sensor 41 incorrespondence with the presence/absence of the reticle R is obtained.The data string of this ratio is stored in the memory at the sameintervals as the digital sampling intervals for the output Sc₁. Thisoutput ratio Sc₁·Sa′/Sc₂·Sa is the true reticle transmittance η obtainedby correcting a detection error due to fluctuations in the illuminanceIp. Since the transmittance η is a function of the position x_(R), itcan be expressed as η(x_(R)). For example, this function can beexpressed by the curve shown in FIG. 5. Referring to FIG. 5, theabscissa indicates the position x_(R) of the reticle in the x direction(scan direction); and the ordinate represents the reticle transmittanceη. Since the position x_(R) changes with time t during a scan operation,η(x_(R))=η(t), provided that the scan operation is performed at aconstant speed. The illuminance Ip is a factor which varies with time.For this reason, in an actual scan/exposure operation, equation (6) ismodified to equation (7), and the illuminance Ip during thescan/exposure operation is sequentially obtained from the output Sa fromthe photoelectric sensor 28 and substituted into equation (7):

P(t)=S×η(t)×Ip(t)tm (7)

η(t)=η(x _(R))

[0079] If the illuminance Ip does not change with time (for example, ifa mercury discharge lamp or the like is used as a light source), avariation in the illuminance Ip during an exposure operation withrespect to one shot area on the wafer W can almost be neglected.Therefore, the illuminance Ip may be detected on the basis of the outputSa from the photoelectric sensor 28 and stored immediately before ascan/exposure operation is started, and Ip(t) can be used as a constantin calculating equation (7). In this case, the illuminance Ip may betreated as a constant value when the shutter is turned on by a shutterON/OFF signal, whereas the illuminance Ip may be treated as Ip=0 whenthe shutter is turned off. In addition, since an output from theradiation amount sensor 41 indicates the incident light amount P(t), theincident light amount P(t) measured before an exposure operation can beused without registering q(t) for each reticle in advance. In any case,since the time t in equation (7) uniquely corresponds to the scanposition of the reticle (or the wafer), the incident light amount P(t)is obtained in real time by reading out the transmittance data η(x_(R))from the memory in accordance with the measurement position x_(R) of theinterferometer 14.

[0080] Furthermore, since the radiation amount sensor 41 is allowed tohave a small light-receiving area as compared with a batch exposure typesensor for illuminating the entire reticle surface at once, aninexpensive, uniform sensor (a silicon photodiode or the like) havingalmost no illuminance irregularity on the light-receiving surface can beused as the radiation amount sensor 41. If the light source 1 is a pulselight source, the radiation amount sensor 41 receives pulse light. Inthis case, the radiation amount sensor 41 may measure the intensity ofeach pulse triggered in accordance with the scan position of the reticleR, and the resulting output Sc may be sequentially loaded as theilluminance Ip. Alternatively, the intensities of pulse light (one or aplurality of pulses) triggered in a predetermined short period of time,e.g., several to several tens of msec may be accumulated, and theaverage illuminance Ip for each period time may be sequentially loaded.

[0081] A method of obtaining the reflectance r in equation (5) will bedescribed next.

[0082] As described above, in addition to light reflected by the wafer Wsurface, light reflected by the reticle R surface or each lens elementof the projection optical system PL is incident on the reflected lightsensor 27. For this reason, the actual wafer reflectance is calculatedin accordance with reference reflection data prepared by using referencereflecting surfaces on the wafer stage WST. Assume that the surface ofthe radiation amount sensor 41 is the reflecting surface R₁ having theknown reflectance r₁, and the surface of the reference plate FM is thereflecting surface R₂ having the known reflectance r₂. The ref lectancesr₁ and r₂ (r₁>0; r₂>0) corresponding to illumination light for exposureat two reference reflecting surfaces are known values measured inadvance, and it is preferable that the two reflectances r₁ and r₂ begreatly different from each other. First, the wafer stage WST is movedsuch that the reflecting surface R₁ is located within the projectedradiation area IA′ while the reticle R is set. The reticle stage RST isthen moved at a predetermined speed while the wafer stage WST is atrest. With this operation, the magnitude of an output I₁ from thereflected light sensor 27 is digitally sampled for each scan position ofthe reticle R, and the sampled data are sequentially stored in thememory of the main control system 100 in correspondence with therespective scan positions. Digital sampling and storage in the memorymay be performed at intervals corresponding to a predetermined movingamount with reference to, e.g., the resolving power (e.g., 0.01 μm) ofthe interferometer 14. In this case, the digital sampling interval neednot coincide with the resolving power of the interferometer 14 and maybe larger than that, e.g., 0.2 μm to 10 μm.

[0083] Subsequently, the wafer stage WST is moved such that thereflecting surface R₂ having the reflectance r₂ is located within theradiation area IA′. The reticle stage RST is then moved at apredetermined speed while the wafer stage WST is at rest. With thisoperation, the magnitude of an output I₂ from the reflected light sensor27 is sequentially stored (digitally sampled) in the memory of the maincontrol system 100 in accordance with each position of the reticle R. Inthis case, the timing of storage in the memory is set to be equal to thedigital sampling interval for the output I₁, and addresses in the memoryare set so that the sampling positions of the outputs I₁ uniquelycorrespond to those of the outputs I₂.

[0084] Especially when the light source 1 is a pulse light source, thevalues of the outputs I₁ and I₂ must be normalized (I₁/Sa; I₂/Sa) byusing the output Sa from the photoelectric sensor 28 to correct anintensity variation of each pulse. This equally applies to the casewherein an ultraviolet line from a mercury discharge lamp is used asillumination light. The normalized values I₁/Sa and I₂/Sa are stored inthe

[0085]FIG. 6 shows the relationship between the output of lightreflected by each reference reflecting surface and the reflectance.Referring to FIG. 6, the values I₁ and I₂ (or I₁/Sa and I₂/sa) sampledwhen the reticle R is moved to a given scan position are plotted alongthe ordinate, and the reflectance is plotted along the abscissa. Asshown in FIG. 6, by drawing a straight line passing coordinates (r₁,I₁)and (r₂,I₂), a reflectance (more accurately, reflection intensity) rx ofthe wafer can be obtained from an output value from the reflected lightsensor 27 which is obtained at this scan position. That is, if theoutput from the reflected light sensor 27, obtained when the reticle Ris moved to the scan position during an actual exposure operation, isrepresented by Ix, the wafer reflectance rx at this time can becalculated according to the following equation by reading out the valuesI₁ and I₂ as the reference reflection data in the memory whichcorrespond to the scan position.

rx=[(r ₂ −r ₁)/(I ₂ −I ₁)]×(Ix−I ₁)+r ₁  (8)

[0086] For example, a method of using three reference reflectingsurfaces having different reflectances and obtaining the straight lineshown in FIG. 6 from three measurement points by the least squareapproximation may be used. In this case, the area of each referencereflecting surface is allowed to be small as compared with a batch typesensor. When the reflected light sensor 27 is to receive pulse light,the intensity of each pulse may be measured, or power may be accumulatedfor a predetermined short period of time, e.g., several to several tensof msec, so as to be output as average power. In any case, the datastrings of the outputs I₁ and I₂ are stored in the memory before anactual exposure operation. Alternatively, equation (8) may be preparedas reference reflection data at each scan position (sampling position)of the reticle R and stored in the memory. As is apparent, when theoutputs I₁ and I₂ are normalized by using the output Sa, the output Ixfrom the reflected light sensor 27, used to obtain the actual waferreflectance rx, is also normalized by using the output Sa andsubstituted into equation (8).

[0087]FIG. 7A shows examples of reference reflection data prepared asoutputs I₁(x_(R)) and I₂(x_(R)) from the reflected light sensor 27,obtained at each scan position of a reticle on the basis of lightreflected by the reference reflecting surfaces, and an output Ix(x_(R))from the reflected light sensor 27, obtained at each position of thereticle on the basis of light reflected by the wafer W during anexposure operation. Referring to FIG. 7A, the ordinate represents theintensity Ix of reflected light; and abscissa represents the positionx_(R) of the reticle in the x directions. Assume that the reticle R isscanned from a position x_(R1) to a position x_(R2). For example,reflectance data rx(x_(R)) corresponding to the scan position of thereticle is calculated according to equation (9) based on equation (8) onthe basis of the output IX(XR) from the reflected light sensor 27,obtained during an actual exposure operation with respect to the firstshot area on the wafer W, the pre-stored data I₁(x_(R)) and I₂(x_(R)),and fixed constants r₁ and r₂. The reflectance data rx(x_(R)) are storedin the memory at the same sampling intervals as the digital samplingintervals for the outputs I₁(x_(R)) and I₂(x_(R)) and at addressesuniquely corresponding to the respective scan positions. FIG. 7B showsthe reflectance data rx(x_(R)) corresponding to the position of thereticle. Referring to FIG. 7B, the ordinate represents the waferreflectance; and the abscissa represents the scan position x_(R) of thereticle in the x direction.

rx(x _(R))=[(r ₂ −r ₁)/(I ₂(x _(R))−I ₁(x _(R)))]×(Ix(x _(R))−I ₁(x_(R)))+r ₁  (9)

[0088] Since the position x_(R) changes with time, if the reticle stageRST is moving at a constant speed during an actual exposure operation,the reflectance data rx(x_(R)) can be replaced with rx(t). Therefore, bysubstituting equations (7) and (9) into equation (5), an energy valueE(t) is calculated by the main control system 100 at the predeterminedtime intervals Δt.

[0089] Calculation of the energy E incident on the projection opticalsystem PL and calculation of a variation in imaging characteristic ofthe projection optical system PL will be described next with referenceto FIGS. 8A and 8B. In this case, for the sake of a simple explanation,the variation ΔM in magnification of the projection optical system PLwill be described hereinafter. FIG. 8A is a graph showing an amount E oflight incident on the projection optical system PL, more specificallyenergies Ea, Eb, and Ec incident on the projection optical system PL.Referring to FIG. 8A, the instantaneous value or average value ofincident energy, obtained at the position of the reticle stage RST atthe predetermined time intervals Δt (e.g., several msec to several tensof msec) is defined as the incident energy E. In FIG. 8A, predeterminedtimings (to be referred to as sampling timings hereinafter) at thepredetermined time intervals Δt are denoted by reference symbols t₁, t₂,t₃, t₄, and t₅, respectively, and the corresponding positions of thereticle stage RST are denoted by reference symbols x₁, x₂, x₃, x₄, andx₅, respectively. It is preferable that measurement of a sampling timebe started when the reticle stage RST reaches the reference position setin storing each type of data described above, and the positions x₁ to x₅coincide with the positions where the respective types of data arestored in the memory. As is apparent, the reticle stage RST iscontrolled to attain a predetermined speed before it reaches thisreference position.

[0090] The main control system 100 calculates energy E(t₁)=Ea which isincident on the projection optical system PL at the sampling timing t₁,as an estimated value, on the basis of a transmittance η(x₁), areflectance rx(x₁), an illuminance Ip(t₁), and the radiation area(determined by the reticle blind 8) IA′ on the wafer W at the samplingtiming t₁ and the position x₁ of the reticle stage RST, according toequations (5), (7), and (9). As described above, if a mercury dischargelamp or the like is used as a light source, opening/closing informationabout the shutter 2 (a weight of “1” is set if the shutter is open; anda weight of “0”, if it is closed) and Ip(t) for Ip=a constant value canbe handled as a constant. Note that if the position x₁ where thetransmittance η(x₁) and the reflectance rx(x₁) are stored does notcorrespond to the sampling timing t₁, a transmittance Δ(x_(R)) and areflectance rx(x_(R)) stored at a position x nearest to the position x₁after the sampling timing t₁ may be used. The opening/closinginformation (1 or 0) about the shutter 2 may be used as follows. If theinformation indicates that the shutter 2 is open at a sampling timing,calculations are executed by using equations (5), (7), and (9) to obtainE(t₁)=Ea. If the information indicates that the shutter is closed,E(t₁)=0 is set without performing calculations according to equations(5), (7), and (9).

[0091] Incident energies are obtained at the sampling timings t₂ to t₅in the same manner as described above. In this case, the incident energyEa is obtained at the sampling timings t₁ and t₃; the incident energy Ebis obtained at the sampling timings t₂ and t₅; and the incident energyEc is obtained at the sampling timing t₄.

[0092] Note that an incident energy may be obtained by using the averagevalue of data obtained at the sampling time intervals Δt (e.g., in thetime interval between the sampling timings t₁ and t₂). Assume that thedigital sampling interval for the transmittance data η(x_(R)) and thereflectance data rx(x_(R)) is set to be 25 μm on the reticle; thesampling time interval Δt between the sampling timings t₁ and t₂ is setto be 5 msec; and a scan speed V is set to be 50 mm/sec. In this case, adistance L the reticle stage moves in the sampling time interval Δt isexpressed as L=V×Δt=250 μm. Since the digital sampling interval for thetransmittance data Δ(x_(R)) and the reflectance data rx(x_(R)) is 25 μm,10 transmittance data η(x_(R)) and 10 reflectance data rx(x_(R)) areobtained as sampled data in the sampling time interval Δt between thesampling timings t₁ and t₂. Hence, the 10 transmittance data η(x_(R))and the 10 reflectance data rx(x_(R)) as the sampled data may beaveraged, respectively, and the resultant data may be used as averagetransmittance data η(x₂) and average reflectance data rx(x₂) at thesampling timing t₂. Subsequently, energy E(t₂)=Eb which is incident onthe projection optical system PL at the sampling timing t₂ is calculatedas an estimated value on the basis of a transmittance η(x₂), areflectance rx(x₂), an illuminance Ip(t₂), opening/closing informationabout the shutter 2 (a weight of “1” is set if the shutter is open; anda weight of “0”, if it is closed), and the area of a radiation area(determined by the reticle blind 8) on the wafer W at the samplingtiming t₂, according to equations (5), (7), and (9). As described above,in this case, if the light source 1 is a light source for emitting pulselight, power in the sampling time interval Δt, as a unit time, betweenthe sampling timings and t₁ and t₂ may be accumulated, and the resultantvalue may be used as average power Ip(t₂) within the unit time. Withregard to the digital sampling interval for the transmittance η(x_(R))and the reflectance rx(x_(R)), since a resolving power smaller than thedistance L the reticle stage moves in the sampling time interval Δt isrequired, the sampling time interval Δt is set such that the distance Lbecomes smaller than the width of the illumination area IA in the scandirection. Note that after the first shot, the incident energy E may beobtained by using the reflectance data rx(x_(R)) stored in the memorywhen the first shot exposure is performed, without obtaining thereflectance rx(x_(R)) according to equation (9).

[0093] Calculation of a variation in optical characteristic of theprojection optical system PL on the basis of the amount of incidentenergy per unit time will be described further with reference to FIG.8B. FIG. 8B shows a magnification variation characteristic ΔMs withrespect to the incident energy E. As shown in FIG. 8B, the magnificationvariation characteristic with respect to the incident energy E isdependent on ΔM/E and the time constant T, as in the case shown in FIG.4B. Therefore, a variation in magnification with respect to incidentenergy at a position corresponding to each time (a predetermined timeinterval) can be obtained from the magnification variationcharacteristic determined by ΔM/E and the time constant T, like the oneshown in FIG. 4B.

[0094] This operation will be described in detail below with referenceto FIG. 8B. The variation ΔM₁ in magnification, caused by the energy Eabetween the sampling timings t₀ and t₁ is obtained from ΔM/E. Asdescribed above, ΔM/E is obtained in advance by an experiment or thelike. Similarly, a variation ΔM₃ in magnification, caused by the energyEb between the sampling timings t₁ and t₂ is obtained from ΔM/E. Thereduction rate of the magnification between the sampling timings t₁ andt₂ is determined by the thermal time constant T so that the reductionamount of the magnification which reduced with time in accordance withthe time constant T can be obtained from the initial value (ΔM₁ in thiscase) between the sampling timings t₁ and t₂. Therefore, the variationin magnification at the sampling timing t₂ is the value obtained bysubtracting the reduction amount between the sampling timings t₁ and t₂from the sum of ΔM₁ and ΔM₂. Similarly, a variation ΔM₃ inmagnification, caused by the energy Ea between the sampling timings t₂and t₃, a variation ΔM₄ in magnification, caused by the energy Ecbetween the sampling timings t₃ and t₄, and a variation ΔM₅ inmagnification, caused by the energy Eb between the sampling timings t₄and t₅, can be obtained from ΔM/E. The reduction amount in each samplinginterval is obtained in the same manner as described above, and thefinal variation in magnification at each sampling timing can beobtained. As a result, an envelope connecting the values at therespective sampling timings can be obtained as a magnification variationcharacteristic, as shown in FIG. 8B. Such calculation methods ofsequentially obtaining a magnification variation characteristic fromdiscrete magnification variation values are disclosed in U.S. Pat. No.4,666,273 and U.S. Pat. No. 4,920,505.

[0095] A method of correcting a magnification will be described next.

[0096] The imaging characteristic controller 30 determines the controlamount of the pressure control system 40 and the driving amounts of thedriving elements 31, 34, and 35 so as to change the magnification inaccordance with the magnification variation characteristic shown in FIG.8B, thereby correcting the magnification. Note that the imagingcharacteristic controller 30 is exclusively used to adjust themagnification M in a direction perpendicular to the scan direction, andthe magnification in the scan direction must be corrected by slightlychanging the moving speed of the reticle R relative to the wafer W.Therefore, the relative speed must be finely adjusted in accordance withthe-adjusting amount of magnification corrected by the imagingcharacteristic controller 30 to isotopically change the size of aprojection image on the entire surface of a shot area.

[0097] The above description is associated with the method of correctinga variation in magnification. Other imaging characteristics can becorrected in the same manner as described above. Note that the patternof the reticle R is sequentially exposed on the wafer W a plurality oftimes. In order to improve the productivity, exposure may be performedby alternately scanning the wafer stage WST (reticle stage RST) inopposite directions in units of shot arrays on the wafer instead ofscanning the stage in one direction all the time. That is, in somecases, after one shot array is exposed, another shot array is exposedwhile the stage is scanned in the opposite direction (i.e., exposure isperformed while the stage is reciprocated). The transmittance data η,the reference reflectance data, and the like described above are storedor calculated in accordance with the position of the reticle R while thereticle R is moved in one direction (e.g., in the −x direction). Forthis reason, if the scan direction of the wafer stage WST is alternatelyreversed in units of shot arrays on a wafer (the scan direction of thereticle stage RST alternately changes to the −x direction and +xdirection), the read direction of the transmittance data η, thereflectance data, and the like is switched in accordance with the scandirection. That is, when a scan operation is to be performed in adirection opposite to the scan direction, of the reticle stage RST, inwhich the transmittance data η and the reference reflectance data arestored, the transmittance data η, the reference reflectance data, andthe like are read out from the memory in the opposite direction.

[0098] In this case, equations (5) and (6) may be used without anymodification by obtaining an average transmittance and an averagereflectance during a scan operation. In this method, however, an averagetransmittance and an average reflectance in one scan operation can onlybe treated as average values, and a reflectance can be calculated onlyafter one scan operation, resulting in a deterioration in precision.Whether a deterioration in precision due to this method falls within anallowable range is determined in consideration of the precision requiredto calculate the variation ΔM in magnification, the variation ΔM inmagnification in one scan operation, the comparison between the timerequired for one scan operation and the time constant T, a change in thetransmittance η of the reticle R with a change in the position of thereticle R, and a change in the reflectance r of the wafer W with achange in the position of the reticle R. However, the time required forone scan operation is dependent on the sensitivity of a resist, and theuniformity of the transmittance and the like of a reticle to be used areindefinite factors. Therefore, in this embodiment, the intensity oflight reflected by a wafer is obtained on the basis of referencereflectance data prepared on the basis of the intensity of lightreflected by each reference reflecting surface in accordance with thescan position of a mask. With this operation, even if the intensity ofreflected light changes in accordance with the position of the reticle,a correct reflectance can be obtained by scanning the reticle during anexposure operation.

[0099] The second embodiment of the present invention will be describednext. The second embodiment is different from the first embodiment inthe following point. In the second embodiment, light amount information(to be referred to as an offset component hereinafter) about lightreflected by a reticle R or an optical member in an illumination opticalsystem is stored in a memory in correspondence with the position of thereticle R, without obtaining reference reflectance data by usingreference reflecting surfaces, and a value obtained by subtracting theoffset component from an output Sb from a reflected light sensor 27 isused as the amount of light which is reflected by a wafer and incidenton a projection optical system PL again. The same reference numerals inthe second embodiment denote the same parts as in the first embodiment.In addition, in this embodiment, information required to obtain theamount of light (light energy) which is incident on the projectionoptical system PL through a reticle, i.e., information about thetransmittance of the reticle (in this embodiment, the transmittance isthe ratio of the amount of light in an illumination area IA to theamount of light which is not shielded by the light-shielding portion ofa pattern but is transmitted therethrough) is detected on the basis ofoutputs from a radiation amount sensor 41 and a light source sensor 28.

[0100] A case wherein the amount of light which is incident on theprojection optical system PL through a reticle is obtained will bedescribed below.

[0101] A main control system 100 stores a ratio Sc/Sa of an output Scfrom the radiation amount sensor 41 to an output Sa from the lightsource sensor 28 in an internal memory in synchronism with an operationof moving a reticle stage RST, on which the reticle R is mounted, by adistance corresponding to one scan operation. That is, the main controlsystem 100 moves the reticle stage RST (while keeping a wafer stage WSTat rest); converts the ratio Sc/Sa of the output Sc from the radiationamount sensor 41 to the output Sa from the light source sensor 28 into atime-series digital value in accordance with the position of the reticlestage RST which is detected by a interferometer 14; and stores thedigital value in the internal memory. This ratio data becomesinformation corresponding to a variation in transmittance in a reticlescan operation. This ratio is denoted by reference symbol Rh. Asdescribed above, storage of data in the memory (digital sampling) may beperformed for each predetermined moving amount (e.g., 0.01 μm to 10 μm)with reference to the resolving power (e.g., 0.01 μm) of theinterferometer 14. The variable ratio Rh of the output Sc from theradiation amount sensor 41 to the output Sa from the light source sensor28, obtained at each stored position of the reticle stage RST, is storedin the memory in correspondence with each position of the reticle stageRST. In an actual exposure operation, the ratio Rh stored in the memoryin advance in correspondence with each position of the reticle stage RSTat predetermined time intervals, e.g., about several msec, is read out,and a value (Sa·Rh) obtained by multiplying the output Sa from thephotoelectric sensor 28 in the actual exposure operation (the outputvalue from the photoelectric sensor 28 at the predetermined timeintervals) by the read value is used an estimated value of the amount oflight (energy) which is incident on the projection optical system PLthrough the reticle at the predetermined time intervals. Since the maincontrol system 100 is constituted by a general digital computer, theratios Rh or the products Sa·Rh may be averaged, and the average valuemay be stored, similar to digital sampling of various types of data inthe first embodiment. Alternatively, the ratios Rh or the products Sa-Rhsequentially calculated with a resolving power almost equal (or lowerthan) the resolving power of the interferometer 14 may be stored withoutany modification.

[0102] Detection of information about the amount of light reflected by awafer will be described below.

[0103] When energy incident on the projection optical system PL is to beconsidered, the amount of light which is reflected by a wafer andincident on the projection optical system PL again must be considered inaddition to the amount of light which is incident on the projectionoptical system PL through a reticle. For this reason, the amount oflight which is reflected by a wafer and incident on the projectionoptical system PL again is detected on the basis of the output Sb fromthe reflected light sensor 27. The main control system 100 moves thereticle stage RST by a distance corresponding to one scan operationwhile the reticle R is mounted on the stage, and stores (digitallysamples) the time-series photoelectric signal Sb (light amountinformation) from the reflected light sensor 27 in the memory inaccordance with the position of the reticle stage RST which is detectedby the interferometer 14. For example, storage of data in the memory maybe performed for each predetermined moving amount with reference to theresolving power (e.g., 0.01 μm) of the interferometer 14. In this case,the digital sampling interval need not coincide with the resolving powerof the interferometer 14 and may be set to be larger than that, e.g.,0.2 μm to 10 μm.

[0104] The output Sb from the reflected light sensor 27 includesinformation about the amount of light reflected by the reticle R andoptical members in the illumination optical system. For this reason, thereticle R is scanned after the reference reflecting surface of areference plate FM having a reflecting surface having an almost zeroreflectance is located within a projection area IA′ of the projectionoptical system PL. In this scan operation, reflected light is receivedby the reflected light sensor 27, and a variation in the output Sb isstored in the memory in accordance with the position of the reticlestage RST. The stored data is used as information about the amount oflight reflected by the reticle R and optical members in the illuminationoptical system. This information will be referred to as an offsetcomponent hereinafter. In an actual exposure operation, the storedoffset component may be subtracted from the output value Sb from thereflected light sensor 27.

[0105] In the above case, if the photoelectric sensor 28, the radiationamount sensor 41 and the reflected light sensor 27 are to receive pulselight, the intensity of each pulse may be detected, or power in a shortperiod of time, e.g., a unit time of several to several tens of msec,may be accumulated, and the resultant value may be output as averagepower in the unit time.

[0106] Calculation of an amount E of light incident on the projectionoptical system PL will be described next with reference to FIGS. 8A and8B.

[0107] The amount E of light incident on the projection optical systemPL and a variation in imaging characteristic of the projection opticalsystem PL can be obtained in the same manner as in the first embodiment.This operation will be briefly described below. In this embodiment,reference symbols Ea, Eb, and Ec in FIG. 8A denote the sums of theamounts of light incident on the projection optical system PL from thereticle side and the amounts of light incident on the projection opticalsystem PL again from the wafer side, with the position of the reticlestage RST being used as a variable. The main control system 100 detectsthe output Sa from the photoelectric sensor 28 and the output Sb fromthe reflected light sensor 27 at a sampling timing t₁. The main controlsystem 100 reads out the output Sa obtained the photoelectric sensor 28at a position x₁ corresponding to the sampling timing t₁, the ratio Rhobtained by the radiation amount sensor 41, and an offset component fromthe memory. The main control system 100 adds the product of the outputSa from the photoelectric sensor 28 and the ratio Rh to a value obtainedby subtracting the offset component corresponding to the position x₁ (orthe timing t₁) from the output Sb from the reflected light sensor 27 Themain control system 100 then calculates an estimated value of energy Eaincident on the projection optical system PL at the sampling timing t₁on the basis of opening/closing information about a shutter 2 (a weightof “1” is set if the shutter is open; and a weight of “0”, if it isclosed), and the area of a radiation area (determined by a reticle blind8) IA′ on the wafer W.

[0108] Note that if the position x₁ where the ratio Rh and the offsetcomponent are stored does not correspond to the sampling timing t₁, theratio Rh and an offset component stored at a nearest position x afterthe sampling timing t₁ may be used.

[0109] Incident energies are obtained at sampling timings t₂ to t₅ inthe same manner as described above. In this case, the incident energy Eais obtained at the sampling timings t₁ and t₃; the incident energy Eb isobtained at the sampling timings t₂ and t₅; and the incident energy Ecis calculated at the sampling timing t₄.

[0110] Note that an incident energy may be obtained by using the averagevalue of data obtained at sampling time intervals Δt (e.g., in the timeinterval between the sampling timings t₁ and t₂), similar to the firstembodiment. Assume that the digital sampling interval for the ratios Rhand offset components is set to be 25 μm on a reticle; the sampling timeinterval Δt between the sampling timings t₁ and t₂ is set to be 5 msec;and a scan speed V is set to be 50 mm/sec. In this case, ratios Rh and10 offset components are obtained as sampled data in the sampling timeinterval Δt between the sampling timings t₁ and t₂. Hence, similar tothe first embodiment, the incident energy Eb may be obtained on thebasis of data obtained by averaging the ratios Rh and the 10 offsetcomponents, respectively.

[0111] When the incident energy is obtained, a variation inmagnification at each sampling timing is obtained from ΔM/E, and thereduction rate of magnification in each sampling time interval isobtained from the time constant T in the same manner as in the firstembodiment. As a result, an envelope connecting the values at therespective sampling timings is set as a magnification variationcharacteristic, thus obtaining the magnification variationcharacteristic shown in shown in FIG. 8B. An imaging characteristiccontroller 30 determines the control amount of a pressure control system40 and the driving amounts of driving elements 31, 34, and 35 so as tochange the magnification in accordance with the magnification variationcharacteristic shown in FIG. 8B, thereby correcting the magnification.

[0112] In this embodiment, the ratios Rh and offset components areloaded by moving the reticle stage RST in one directions. For thisreason, when the reticle stage RST is to be scanned in a directiondifferent from the loading direction of the ratios Rh and the offsetcomponents, these data must be read out in the opposite direction.

[0113] In the first and second embodiments, information about thetransmittance of a reticle and information about light reflected by awafer are stored in accordance with the coordinate position of thereticle. However, since the wafer stage WST is scanned at the same time,the same effect as that described above can be obtained even if thesepieces of information are stored with reference to the coordinateposition of the wafer stage or time. When storage of data is to beperformed with reference to the coordinate position, an interferometercounter must be reset to “0” at the start of a scan operation, or thecoordinate position at the start of a scan operation must be stored.When storage of data is to be performed with reference to time, the timebase scale needs to be changed because the scan speed changes with achange in exposure time owing to the sensitivity of a resist. Note thatalthough the precision slightly deteriorates, in the above embodiments,the variation characteristic of imaging characteristic of the projectionoptical system PL may be obtained on the basis of only the amount oflight which is incident on the projection optical system PL through areticle.

[0114] When the illumination condition is changed upon replacement of anaperture stop 29, the passing position of a light beam in the projectionoptical system PL changes, and hence the variation characteristic ofimaging characteristic changes. For example, a thermal time constant andthe like associated with a variation in magnification change. Therefore,information (e.g., a thermal time constant) about the variationcharacteristic of imaging characteristic must be replaced every time theillumination condition is changed upon replacement of the aperture stop29.

[0115]FIG. 9 shows the positional relationship between the reticle blind8 viewed from above, a projection field if, and a pattern area PA of thereticle R. In this case, the reticle blind 8 is constituted by twolight-shielding plates 8A and 8B. The light-shielding plate 8B has a Ushape when viewed from above. The light-shielding plate 8B has astraight edge EGx₂ defining an illumination area in the scan direction(x direction), and straight edges EGy₁ and EGy₂ defining theillumination area in the y direction perpendicular to the scandirection. The light-shielding plate 8A has a straight edge EGx₁parallel to the edge EGx₂ of the light-shielding plate 8B to define theillumination area in the scan direction. The light-shielding plate BA isdesigned to be movable in the x direction with respect to thelight-shielding plate 8B. With this structure, the width of theslit-like illumination area IA can be changed in the scan direction. Thelight-shielding plate 8B may also be designed to be translated in the xdirection such that the edges EGx₁ and EGx₂ defining the illuminationarea in the scan direction are set to be symmetrical with respect to anoptical axis IX. FIG. 10 is a perspective view stereoscopically showingthe intensity distribution of illumination light which is incident onthe reticle R through the reticle blind 8 shown in FIG. 9. Referring toFIG. 10, a direction along the optical axis IX is defined as anintensity axis I. No significant problem is posed when a continuouslight source such as a mercury discharge lamp is used as an illuminationlight source. However, when a pulse light source is to be used, if theilluminance distribution in the scan direction exhibits a normalrectangular shape, exposure light amount irregularity tends to occur inone shot area on the wafer W because of variations in superpositionamount or in the number of times of superposition at two end portions ofthe illuminance distribution in the scan direction.

[0116] For this reason, as shown in FIG. 10, at least end portions ofthe illuminance distribution in the scan direction are caused to havealmost uniform inclinations (width ΔXs). Referring to FIG. 10, a lengthYSp of the illuminance distribution in the y direction is set to coverthe length of the pattern area PA of the reticle R in the y direction,and a length (slight width) XSp of the illuminance distribution in the xdirection is optimally determined in consideration of a target exposurelight amount for the photoresist on the wafer W, the scan speeds of thereticle stage RST and the wafer stage WST, the pulse oscillationfrequency of a pulse light source (if it is used), the intensity ofillumination light, and the like. As shown in FIG. 10, in order toincline the two ends of the illuminance distribution by the width ΔXs,the edge EGx₁ of the light-shielding plate 8A and the edge EGx₂ of thelight-shielding plate 8B in FIG. 9 may be shifted from a positionconjugate to the pattern surface of the reticle R in a direction alongthe optical axis IX by a predetermined amount so as to project slightlydefocused images of the edges EGx₁ and the EGX₂ onto the reticle R.When, however, sharp images of the edges EGy₁ and EGy₂ in a non-scandirection are to be formed on the pattern surface of the reticle R, theedges EGy₁ and EGy₂ must be accurately located at a position conjugateto the pattern surface of the reticle R. For this reason, the edges EGy₁and EGy₂ are accurately located within a conjugate plane, and the edgesEGx₁ and EGx₂ are located within a plane slightly shifted from the planeposition of the edges EGy₁ and EGy₂ to the light source side. Inaddition, in order to variably change the longitudinal dimension (lengthYSp) of the slit-like illumination area IA, the edges EGy₁ and EGy₂ mustalso be designed to be movable in the y direction. If the illuminancedistribution shown in FIG. 10 is uniformly inclined in the y direction,as indicated by an imaginary line LLi, the exposure light amount at aportion of a shot area which is exposed at a position ya₁ in the ydirection differs from that at a portion of the shot area which isexposed at a position ya₂. For this reason, it is preferable that anintensity I(ya₁) at the position ya₁ and an intensity I(ya₂) at theposition ya₂ be measured to finely adjust the slit width XSp in the ydirection in accordance with a ratio I(ya₁)/I(ya₂). Let XSp(ya₁) be thewidth of the slit-like illumination area IA in the scan direction at theposition ya₁ in the y direction, and XSp(ya₂) be the width in the scandirection at the position ya₂. In this case, the edges EGx₁ and EGx₂ areinclined (rotated) relative to each other from the parallel state withinthe x-y plane so that I(ya₁)/I(ya₂)=XSp(ya₂)/XSp(ya₁)) is establishedthat is, the slit-like blind opening shown in FIG. 9 is formed into aslightly trapezoidal shape. With this arrangement, an accurate amount ofexposure light can be given to each point in a shot area even withslight illuminance irregularity (uniform inclination) of slit-likeillumination light in a non-scan direction.

[0117] When a pulse light source is to be used, pulse emission must beperformed with a specific positional relationship while the reticle Rand the wafer W are relatively scanned. FIG. 11 illustrates illuminancecharacteristics in the scan direction when pulse emission is performedwith the specific positional relationship. In pulse emission, since thepeak intensity value of each pulse varies, pulse emission (triggeroperation) is performed at intervals of a distance into which the width(XPs+ΔXs) of the slit-like illumination area IA in the scan directioncan be divided by a predetermined integer value Np (excluding 1) whenthe illumination area IA is defined by an intensity Im/2 where Im is theaverage value of the intensities of pulse light. Assume that the width(XPs+ΔXs) of the slit-like illumination area IA on the reticle is 8 mm,and the integer value Np is 20. In this case, the pulse light source maybe caused to emit pulse light every time the reticle R is scanned/movedby 0.4 mm. The integer value Np is the number of pulses superposed at anarbitrary point on the wafer W. Therefore, in order to achieve a desiredexposure precision on a wafer by averaging variations in peak intensityvalue of each pulse, the minimum value of the integer value Np isautomatically determined in accordance with the variations in intensityof each pulse. The minimum value of the integer value Np is expected tobe about 20 from the performance of an existing pulse light source(e.g., an excimer laser).

[0118] Referring to FIG. 11, since the integer value NP is set to be 5,the inclination of the trailing end portion of the illuminancedistribution of the first pulse in the scan direction overlaps theinclination of the leading end portion of the illumination distributionof the sixth pulse in the scan direction. In addition, at the start orend of a scan/exposure operation, pulse oscillation is started from astate wherein the entire slit-like illumination area IA (width:XPs+2ΔXs) is located outside the pattern area PA of the reticle R, andthe pulse oscillation is stopped when the entire illumination area IA(width: XPs+2ΔXs) reaches the outside of the pattern area PA.

[0119] Two methods of triggering a pulse light source can be considered.One method is a position synchronization trigger method of supplying atrigger signal to the pulse light source for a predetermined movingamount in response to a measurement value obtained by the laserinterferometer 14 (or 19) for measuring the position of the reticlestage RST (or the wafer stage WST) in the scan direction. The othermethod is a time synchronization trigger method of generating clocksignals at predetermined time intervals (e.g., 2 msec) based on theconstant speeds of the reticle stage RST and the wafer stage WST,assuming that constant speed control therefor is reliable, and supplyingthe signals, as trigger signals, to the pulse light source. The twomethods have their own merits and demerits and hence may be selectivelyused. In the time synchronization trigger method, however, thegeneration start timing and stop timing of clock signals must bedetermined in response to measurement values obtained by the laserinterferometer 14 or 19.

[0120] If the highest priority is given to the minimization of theexposure processing time for one shot area, the speeds of the reticlestage RST and the wafer stage WST, the width (XPs) of the slit-likeillumination area IA, and the peak intensity of pulses are preferablyset so that the pulse light source oscillates at about the rated maximumoscillation frequency (a predetermined maximum frequency), provided thata target exposure amount can be obtained.

[0121] Furthermore, as described in each embodiment, when various dataare to be formed by sampling the outputs Sa and Sb from thephotoelectric sensor 28 and the reflected light sensor 27 while scanningonly the reticle R, or when the pulse light source is oscillated by thetime synchronization trigger method, the outputs Sa and Sb during ascan/exposure operation may be sampled in response to trigger clocksignals.

1. A scanning exposure apparatus in which a substrate is exposed whilemoving a mask in a first direction and moving the substrate in a seconddirection, the apparatus comprising: a projection system disposed in apath of an exposure beam and which projects an image of a pattern of themask onto the substrate, the mask being to be provided on one side ofthe projection system and the substrate to be provided on the other sideof the projection system; a mask stage disposed on the one side of theprojection system and which is movable while holding the mask; asubstrate stage disposed on the other side of the projection system andwhich is movable while holding the substrate; a first interferometersystem optically connected to the mask stage and which measurespositional information of the mask stage; and a second interferometersystem optically connected to the substrate stage and which has fivemeasurement axes including two first measurement axes parallel to thesecond direction and two second measurement axes perpendicular to thesecond direction, and which measures positional information of thesubstrate stage.
 2. An apparatus according to claim 1 , furthercomprising: a calculator operatively connected to the secondinterferometer system and which acquires rotational information of thesubstrate stage using said two first measurement axes or said two secondmeasurement axes.
 3. An apparatus according to claim 2 , wherein saidcalculator uses said two first measurement axes.
 4. An apparatusaccording to claim 2 , wherein said calculator uses said two secondmeasurement axes.
 5. An apparatus according to claim 1 , furthercomprising: a calculator operatively connected to the secondinterferometer system and which acquires positional information of thesubstrate stage in the second direction by calculating an average ofmeasurement values obtained with the two first measurement axes.
 6. Anapparatus according to claim 1 , further comprising: a calculatoroperatively connected to the second interferometer system and whichacquires positional information of the substrate stage in a directionperpendicular to the second direction by calculating an average ofmeasurement values obtained with said two second measurement axes.
 7. Anapparatus according to claim 1 , wherein said two first measurement axesare symmetrical with respect to a straight line which passes through acenter of a projection field of a projection system.
 8. An apparatusaccording to claim 1 , wherein said two second measurement axes aresymmetrical with respect to a straight line which passes through acenter of a projection field of said projection system.
 9. An apparatusaccording to claim 1 , further comprising: a beam source, which emitspulses of an exposure beam substantially at a rated maximum frequency.10. An apparatus according to claim 1 , further comprising: an adjustingsystem operatively connected to the first interferometer system or thesecond interferometer system and which adjusts an imaging characteristicof the image to be projected onto the substrate based on the positionalinformation in the first direction from the first interferometer systemor the positional information in the second direction from the secondinterferometer system.
 11. A scanning exposure method in which ascanning exposure is performed while moving a mask in a first directionand moving a substrate in a second direction, the method comprising:measuring, during the scanning exposure, positional information of themask using a first interferometer system; and measuring, during thescanning exposure, positional information of the substrate using asecond interferometer system which has five measurement axes includingtwo first measurement axes parallel to the second direction and twosecond measurement axes perpendicular to the second direction.
 12. Amethod according to claim 11 , further comprising: acquiring rotationalinformation of the substrate using said two first measurement axes orsaid two second measurement axes.
 13. A method according to claim 12 ,wherein said rotational information is acquired using said two firstmeasurement axes.
 14. A method according to claim 12 , wherein saidrotational information is acquired using said two second measurementaxes.
 15. A method according to claim 12 , wherein said acquiringincludes calculating an average of first rotational information obtainedwith said two first measurement axes and second rotational informationobtained with said two second measurement axes.
 16. A method accordingto claim 12 , further comprising: acquiring positional information ofthe substrate in the second direction, including calculating an averageof measurement values obtained with said two first measurement axes. 17.A method according to claim 12 , further comprising: acquiringpositional information of the substrate in a direction perpendicular tothe second direction, including calculating an average of measurementvalues obtained with said two second measurement axes.
 18. A methodaccording to claim 11 , further comprising: acquiring positionalinformation of the substrate in the second direction, includingcalculating an average of measurement values obtained with said twofirst measurement axes.
 19. A method according to claim 18 , furthercomprising: acquiring positional information of the substrate in adirection perpendicular to the second direction, including calculatingan average of measurement values obtained with said two secondmeasurement axes.
 20. A method according to claim 11 , furthercomprising: acquiring positional information of the substrate in adirection perpendicular to the second direction, including calculatingan average of measurement values obtained with said two secondmeasurement axes.
 21. A method according to claim 11 , wherein said twofirst measurement axes are symmetrical with respect to a straight linewhich passes through a center of a projection field of a projectionsystem through which the substrate is exposed with a pattern of themask.
 22. A method according to claim 21 , wherein said two secondmeasurement axes are symmetrical with respect to a straight line whichpasses through the center of the projection field of said projectionsystem.
 23. A method according to claim 11 , wherein said two secondmeasurement axes are symmetrical with respect to a straight line whichpasses through a center of a projection field of a projection systemthrough which the substrate is exposed with a pattern of the mask.
 24. Amethod according to claim 11 , further comprising: emitting pulses of anexposure beam substantially at a rated maximum frequency during thescanning exposure.
 25. A method according to claim 24 , furthercomprising: determining a scan speed of the substrate such that saidsubstrate is supplied with a target exposure dose by irradiation withthe pulses of the exposure beam emitted substantially at the ratedmaximum frequency.
 26. A method according to claim 25 , furthercomprising: adjusting an intensity of said pulses of the exposure beamsuch that said substrate is supplied with a target exposure dose byirradiation with the pulses of the exposure beam emitted substantiallyat the rated maximum frequency.
 27. A method according to claim 25 ,further comprising: adjusting a width of an irradiation area of saidpulses of the exposure beam in a scanning direction of said substratesuch that said substrate is supplied with a target exposure dose byirradiation with the pulses of the exposure beam emitted substantiallyat the rated maximum frequency.
 28. A method according to claim 11 ,further comprising: adjusting, during the scanning exposure, an imagingcharacteristic of an image to be projected onto the substrate.
 29. Amethod according to claim 28 , wherein said adjusting includes movingthe mask.
 30. A method according to claim 11 , further comprising:adjusting a relative speed between the mask and the substrate to adjusta magnification in the second direction of a pattern transferred ontothe substrate; and adjusting a projection system which projects an imageof a pattern of said mask onto the substrate to adjust a magnificationin a direction perpendicular to the second direction of the patterntransferred onto the substrate.
 31. A device manufacturing methodincluding an exposure process in which a scanning exposure of asubstrate is performed while moving a mask in a first direction andmoving the substrate in the second direction, the method comprising:measuring, during the scanning exposure, positional information of themask using a first interferometer system; and measuring, during thescanning exposure, positional information of the substrate using asecond interferometer system which has five measurement axes includingtwo first measurement axes parallel to the second direction and twosecond measurement axes perpendicular to the second direction.